1. Field of the Invention
The invention relates to a carrier starch for corrugating adhesives wherein modified starch is preblended with dry alkali, to make a preblended carrier starch which eliminates the need for caustic at the corrugator and exhibits improved viscosity and storage stability over time. The dry alkali also obviates the need to add silicate compounds in a corrugating adhesive composition. Specifically, the present invention is a preblended carrier starch comprising a modified starch, such as oxidized starch, and a dry alkali such as sodium metasilicate or trisodium phosphate.
2. Description of the Related Art
Adhesives used in manufacturing corrugated board are usually comprised of starch, caustic, a boron containing compound and, where water resistance is needed, a waterproofing resin. Adhesives are also known in the art which comprise an alkali, such as a silicate. The main binder of corrugating paper is the starch which is gelatinized in the corrugating process as it penetrates the paper fiber. The other components, namely, the caustic, boron containing compounds, and waterproofing resin are auxiliary agents which modify the basic properties of the starch.
Caustic, usually in the form of sodium hydroxide, directly affects the gelatinization point temperature of the starch. Caustic also enhances penetration of the gelatinized starch into the surface paper fibers allowing for better bonding.
The boron containing compounds perform multiple tasks. The most important function is developing the adhesive tackiness that is crucial in the formation of the green bond. The green bond is the bond which holds the components of the corrugated board together until final heat curing of the adhesive system. It relies solely on the viscosity of the adhesive to maintain the integrity of the product during curing. Corrugating machine speeds therefore are limited by the rate of viscosity increase (i.e., rate of green bond formation) in the bond line between the liner and the corrugated medium. The oxygen atoms attached to the boron form stronger bonds, sometimes called boron bridges, between the starch and cellulose hydroxyl groups. Boron containing compounds also act as buffering agents in the presence of caustic and help maintain the viscosity stability of the adhesive paste.
Water proof or water resistant resins are considered to be an optional component of adhesives, and although some corrugating plants may not need them, most plants are prepared to use them when needed. Many types of thermo-setting resins are suitable. The most common of them are derived from urea-formaldehyde, ketone-formaldehyde or melamine-formaldehyde. When heat and pressure is applied to them in a corrugating machine, they will condense to form an excellent water resistant network.
In the corrugating process, adhesive is commonly applied to the tips of the flutes of a corrugated medium. Then a noncorrugated flat paper liner is applied against the adhesive coated flutes as they pass between a corrugating roll and a pressure roll. The resulting product has the corrugating medium on one side and a flat liner on the other side and is called a single-faced portion. The single-faced portion may be used "as is" (called a "single facer" board) or adhesive may be applied to the flute tips of the single-faced portion and a second flat sheet can be applied in the same manner as the first in what is called a "double-facer" or a "double-backer" operation. The second liner sheet is treated with heat and reduced pressure (relative to the pressure used to make single-faced portion) immediately following contact with the adhesive.
Starch-based adhesives which can be of the carrier, no-carrier and carrier-no-carrier type are commonly used in processes for manufacturing corrugated paper board. In carrier type adhesives, such as those relating the present invention, a portion of the starch forms a carrier, often known as the gelatinized phase, which suspends the balance of the starch which is in an ungelatinized state. Under conditions of heat and pressure, the ungelatinized starch is rapidly hydrated and gelatinized to increase quickly the viscosity and adhesivity of the adhesive composition. In no-carrier type adhesives, all of the starch is slightly cooked or swollen with heat and caustic soda for viscosity. Finally, carrier-no-carrier type adhesives have a portion of the starch which forms a carrier and is responsible for about one half of the viscosity and the remaining viscosity is obtained by slightly swelling the uncooked starch.
With carrier type and carrier-no-carrier type adhesives, the carrier portion of the adhesive composition can be said to exist separately from the adhesive. As such, a carrier starch may be added by the end user to the other components of the adhesive at the corrugator or the carrier may be added to the other components of the adhesive prior to end use at the corrugator. The invention involves carrier starches which can be added to other components to form an adhesive either at the corrugator or by the end user prior to the corrugating process. The carrier starches of the present invention exhibit enhanced storage stability, and in particular excellent viscosity stability over time. Thus, the carrier starches of the invention are superior for use at the corrugator, or prior to the corrugating process, depending on the needs and requirements of the end user.
U.S. Pat. No. 5,571,316 issued to Gill describes silicate-dextrin adhesives comprising sodium metasilicate; dextrin; alkali metal silicate, such as sodium silicate or potassium silicate; sodium metaborate octhydrate or sodium metaborate tetrahydrate and water. The adhesive compositions in U.S. Pat. No. 5,571,316 are particularly useful in making paper tubes, corrugated boards and angle boards wherein layers of paper are adhered to each other by the adhesive formulation. Dextrin is, generally, treated with acid to make it soluble and useful as a component of adhesive formulations. Acid treatment of dextrin, however, results in low viscosity and, consequently, adhesives made from dextrin require bulking to increase the solids content acceptable for corrugating adhesives. Accordingly, Gill teaches that dextrin-silicate adhesive formulations require solids content of from about 30% by weight to about 44% by weight.
Adhesive compositions comprising sodium silicates and clay are known in the art. For example, U.S. Pat. No. 2,669,282 issued to Kreyling, describes a wet adhesive composition comprising sodium silicate, starch and siliceous material such as kaolinitic clay or diatomaceous earth. These adhesive compositions, however, are quite distinct from the starch/dry alkali preblended carrier starches of the present invention.
Canadian Patent 1056107 granted to PQ Corporation, U.S.A. concerns starch-silicate adhesive compositions. These adhesive compositions can be prepared with 8% to 40% of the solids as alkali metal silicate and 60% to 92% of the solids as starch. These adhesive compositions do not involve the use of most modified starches, and generally require caustic and silicate.
It has now been found that preblending modified starch, particularly oxidized starch, with a dry alkali produces a carrier starch for corrugating adhesives which eliminates the need for a caustic at the time that the adhesive is prepared for use at the corrugator by allowing for an adjustment of the carrier to an appropriate pH to promote swelling or gelling of the starch. The preblended carrier starches of the invention also do not require added silicate, such as a metal silicate source, in the carrier phase or for a corrugating adhesive composition formulated with the carrier starch. It has now been found that the use of modified starches in accordance with the invention and, in particular, oxidized starch enhances the viscosity stability of the adhesive compositions over time and temperature fluctuations.
It is further found that sodium metasilicate has unexpected advantages compared with calcium salts, as the dry alkali in the blend, because sodium metasilicate eliminates the crosslinking of the starch in the carrier type adhesive that is experienced when calcium salts are used as the dry alkali. Trisodium phosphate is also a useful dry alkali in the invention. The buffering effect of the dry alkali salt, such as sodium metasilicate or trisodium phosphate, also reduces the effect from water ions and dissolved solids on adhesive viscosity.
It is also found that the use of a dry alkali in the carrier starches of the invention advantageously makes it unnecessary to employ two conventional chemicals for end use or at the corrugator. Accordingly, the dry alkali allows for elimination of the need for added caustic at the corrugator or when the carrier starch of the present invention is prepared for use. The dry alkali, particularly sodium metasilicate, in the carrier starch, can also replace the use of additional metal silicates in the carrier phase or in the adhesive composition.
In the present specification and claims, all parts and percentages are by weight/weight unless otherwise specified.